Non-reciprocal circuit devices such as isolators are used in mobile communications equipments utilizing frequencies from several hundreds MHz to several tens GHz, such as base stations and terminals of cell phones, etc. In transmission systems of mobile communications equipments, for instance, isolators are disposed between power amplifiers and antennas to prevent unnecessary signals from returning to the power amplifiers, thereby stabilizing the impedance of the power amplifiers on the load side. Accordingly, the isolators are required to have excellent insertion loss characteristics, reflection loss characteristics and isolation characteristics.
Conventionally known as such isolators is a three-terminal isolator shown in FIG. 26. This isolator comprises three central conductors 31, 32, 33 crossing at an angle of 120° with electric insulation on one main surface of a ferrimagnetic microwave ferrite 38, each central conductor 31, 32, 33 having one end connected to the ground and the other end connected to a matching capacitor C1-C3, and a terminal resistor Rt is connected to a port (for instance, P3) of one of the central conductors 31, 32, 33. A DC magnetic field Hdc is axially applied from a permanent magnet (not shown) to the ferrite 38. This isolator transmits high-frequency signals from a port P1 to a port P2, while absorbing reflection waves from the port P2 by the terminal resistor Rt to prevent them from being transmitted to the port P1, thereby preventing unnecessary reflection waves generated by the impedance variation of an antenna from entering a power amplifier, etc.
Attention has recently been getting paid to a two-port isolator comprising two central conductors and having excellent insertion loss characteristics and reflection characteristics (JP 2004-88743 A). FIG. 27 shows a equivalent circuit of the two-port isolator, and FIG. 28 shows its structure.
This two-port isolator 1 comprises a central electrode L1 (first inductance element) electrically connected between first and second input/output ports P1, P2, a central electrode L2 (second inductance element) crossing the central electrode L1 with electric insulation and electrically connected between the second input/output port P2 and the ground, a capacitance element C1 electrically connected between the first and second input/output ports P1, P2 to constitute a first parallel resonance circuit with the central electrode L1, a resistance element R, and a capacitance element C2 electrically connected between the second input/output port P2 and the ground to constitute a second parallel resonance circuit with the central electrode L2. A frequency providing the maximum isolation (attenuation in reverse direction) is set by the first parallel resonance circuit, and a frequency providing the minimum insertion loss is set by the second parallel resonance circuit. When high-frequency signals are transmitted from the first input/output port P1 to the second input/output port P2, resonation occurs not in the first parallel resonance circuit between the first and second input/output ports P1, P2, but in the second parallel resonance circuit, resulting in small transmission loss and good insertion loss characteristics. Current inversely flowing from the second input/output port P2 to the first input/output port P1 is absorbed by the resistance element R connected between the first and second input/output ports P1, P2.
As shown in FIG. 28, the two-port isolator 1 comprises metal cases (upper case 4 and lower case 8) made of a ferromagnetic material such as soft iron, etc. to constitute a magnetic circuit, a permanent magnet 9, a central conductor assembly 30 comprising a microwave ferrite 20 and central conductors 21, 22, and a laminate substrate 50 on which the central conductor assembly 30 is mounted. Each case 4, 8 is plated with a conductive metal such as Ag, Cu, etc.
The central conductor assembly 30 comprises a disk-shaped microwave ferrite 20, and central conductors 21, 22 perpendicularly crossing thereon via an insulating layer (not shown). The central conductors 21, 22 are electromagnetically coupled to each other in crossing portions. Each central conductor 21, 22 is constituted by two lines, both end portions thereof separately extending along a lower surface of the microwave ferrite 20.
As shown in FIG. 29, the laminate substrate 50 comprises connecting electrodes 51-54 connected to end portions of the central conductors 21, 22, a dielectric sheet 41 having capacitor electrodes 55, 56 and a resistor 27 on the rear surface, a dielectric sheet 42 having a capacitor electrode 57 on the rear surface, a dielectric sheet 43 having a ground electrode 58 on the rear surface, and a dielectric sheet 45 having an external input electrode 14, an external output electrode 15 and external ground electrodes 16. The connecting electrode 51 acts as the first input/output port P1, and the connecting electrodes 53, 54 act as the second input/output port P2.
The central conductor 21 has one end electrically connected to an external input electrode 14 via the first input/output port P1 (connecting electrode 51), and the other end electrically connected to an external output electrode 15 via the second input/output port P2 (connecting electrode 54). The central conductor 22 has one end electrically connected to an external output electrode 15 via the second input/output port P2 (connecting electrode 53), and the other end electrically connected to an external ground electrode 16. A capacitance element C1 is electrically connected between the first input/output port P1 and the second input/output port P2 to constitute a first parallel resonance circuit with the central conductor L1. A capacitance element C2 is electrically connected between the second input/output port P2 and the ground to constitute a second parallel resonance circuit with the central conductor L2
Cell phones have become handling wider frequency bands (wideband), and pluralities of transmission/receiving systems such as WCDMA, PDC, PHS, GSM, etc. (multi-band, multi-system, etc.) to adapt to increasing numbers of users. Accordingly, non-reciprocal circuit devices have been getting required to be operable in wider frequency bands. One of data transmission technologies, which uses a cell phone network for GSM and TDMA systems, is an enhanced data GSM environment (EDGE). When two bands of GSM850/900 are used, a frequency passband required for the non-reciprocal circuit device is 824-915 MHz.
To obtain a wideband, non-reciprocal circuit device, various factors of causing unevenness, such as inductance generated in lines connecting reactance elements, floating capacitance generated by interference between electrode patterns, etc., should be taken into consideration. In the two-port isolator, however, unnecessary reactance components are connected to the first and second parallel resonance circuits, resulting in the deviation of the input impedance of the two-port isolator from the desired level. As a result, there appears impedance mismatching between the two-port isolator and the other circuits connected thereto, leading to deteriorated insertion loss and isolation characteristics.
Although it is not impossible to determine inductance and capacitance in the first and second parallel resonance circuits taking unnecessary reactance components into consideration, it would be difficult to separately adjust the input impedance of the first and second input/output ports P1, P2 if the width, gap, etc. of lines forming the central conductors 21, 22 were simply changed, so that it has been practically difficult to obtain the optimum conditions of matching with external circuits. This is because the central conductors 21, 22 are coupled to each other, the change of the width, gap, etc. of lines would result in changing the inductance of the first and second inductance elements L1, L2. Particularly deviation in the input impedance of the first input/output port P1 should be avoided because it increases the insertion loss.